In general, for automobiles, household electrical appliances, and other applications requiring excellent workability, for example, as disclosed in Japanese Patent Publication (B) No. 42-12348 and Japanese Patent Publication (B) No. 54-12883, ultralow carbon steel having a C concentration of 0.015 mass % or less and including Ti, Nb, and other strong carbide forming elements are being broadly used. Attempts have been made to further improve workability up to now by improving the method of production. Further, Japanese Patent Publication (A) No. 3-170618 and Japanese Patent Publication (A) No. 4-52229 propose steel sheet excellent in deep drawability, stretch formability, and other aspects of workability by increasing the sheet thickness in the final hot rolling or raising the hot rolled sheet coiling temperature. However, the problem has arisen that the increasing harshness of the hot rolling conditions increases the load on the heating furnace and hot rolling machine.
In the above ultralow carbon steel including Ti or Nb, fine carbides are present in the steel, so recrystallization is remarkably suppressed. For this reason, annealing at a high temperature becomes necessary. There are also issues such as the occurrence of heat buckling or sheet breakage during rolling and the increase in the amount of energy consumption. As opposed to this, as shown in Japanese Patent Publication (A) No. 6-212354 and Japanese Patent Publication (A) No. 6-271978, steel sheet with a low recrystallization temperature has been developed by setting suitable amounts of Mn and P in ultralow carbon steel not containing Nb or Ti and changing the hot rolling conditions. However, in these inventions, Mn or P is added in large amounts, so the alloy cost rises and therefore obtaining steel sheet for ultradeep drawing of a total elongation or 50% or more and a Lankford value (r value) of 2.0 or more is difficult. Further, ultralow carbon steel sheet usually is produced by deoxidizing by Al not yet deoxidized molten steel decarburized to the ultralow carbon range in a vacuum degassing system (RH) etc., that is, is “Al killed steel”, so the molten steel contains a large amount of alumina inclusions. These alumina inclusions easily coalesce and join together in the molten steel and remain in the cast slab as large alumina clusters, so at the time of hot rolling and cold rolling, the alumina clusters become exposed at the steel sheet surface and cause surface defects. Further, when the alumina clusters remain inside the steel sheet, they become the cause of cracks, defects, and other flaws at the time of press forming. The formability also sharply falls.
In particular, in ultralow carbon steel, if the workability becomes better, the susceptibility to surface defects or cracks rises and even if going to the trouble of developing steel sheet with excellent workability, the yield obtained as a product is low and a large cost increase is incurred. To deal with these problems accompanying Al deoxidation, for example, as shown in
Japanese Patent Publication (A) No. 61-276756 and Japanese Patent Publication (A) No. 58-185752, the method has been proposed of treating molten steel by Ca to convert the alumina clusters to low melting point calcium aluminate for quick removal by floatation. However, conversion of alumina clusters requires a large amount of Ca. It is known that Ca reacts with the S in the steel to form CaS and becomes a cause of rusting. Further, as shown in Japanese Patent Publication (A) No. 10-226843, the method has also been developed of adding fine amounts of Al and Ti for deoxidation and controlling the inclusions in the molten steel to inclusion compositions with good crushability mainly comprised of Ti oxides, Mn oxides, Si oxides, and alumina.
However, molten steel contains dissolved Al, so if the molten steel is reoxidized by the slag or air, the composition of titania-based inclusions caused by Ti deoxidation changes to the high alumina side and results in aggregation and coarsening, so this is not a fundamental resolution of the problems of surface defects and press defects. Further, the Mn oxides, Si oxides, and Ti oxides have to be made complex, but the upper limit value of the amount of addition of Ti is low, so there was the problem that a high workability material could not necessarily be obtained.